Motion compensated inter-frame coding has been widely adopted in various coding standards, such as MPEG-1/2/4 and H.261/H.263/H.264/AVC. While motion-compensated inter-frame coding can effectively reduce bitrate for compressed video, intra coding is required to compress the regions with high motion or scene changes. Besides, intra coding is also used to process an initial picture or to periodically insert I-pictures or I-blocks for random access or for alleviation of error propagation. Intra prediction exploits the spatial correlation within a picture or within a picture region. In practice, a picture or a picture region is divided into blocks and the intra prediction is performed on a block basis. Intra prediction for a current block can rely on pixels in neighboring blocks that have been processed. For example, if blocks in a picture or picture region are processed row by row first from left to right and then from top to bottom, neighboring blocks on the top and neighboring blocks on the left of the current block can be used to form intra prediction for pixels in the current block. While any pixels in the processed neighboring blocks can be used for intra predictor of pixels in the current block, very often only pixels of the neighboring blocks that are adjacent to the current block boundaries on the top and on the left are used.
The intra predictor is usually designed to exploit spatial features in the picture such as smooth area (DC mode), vertical line or edge, horizontal line or edge and diagonal line or edge. Furthermore, spatial correlation often exists between the luminance (luma) and chrominance (chroma) components. Therefore, reconstructed luma pixels can be used to derive the intra chroma prediction. In recent development of High Efficiency Video Coding (HEVC), a chroma intra prediction method based on co-located reconstructed luma blocks has been disclosed. The type of chroma intra prediction is termed as LM prediction. The main concept is to use the reconstructed luma pixels to generate the predictors of corresponding chroma pixels. FIG. 1 illustrates the prediction procedure. First, the neighboring reconstructed pixels of a co-located luma block in FIG. 1A and the neighboring reconstructed pixels of a chroma block in FIG. 1B are used to derive the correlation parameters between the blocks. Then, the predicted pixels of the chroma block are generated using the parameters and the reconstructed pixels of the luma block. In the parameters derivation, the first above reconstructed pixel row and the second left reconstructed pixel column of the current luma block are used. The specific row and column of the luma block are used in order to match the 4:2:0 sampling format of the chroma components.
According to the LM prediction mode, the chroma values are predicted from reconstructed luma values of a co-located block. The chroma components may have lower spatial resolution than the luma component. In order to use the luma signal for chroma intra prediction, the resolution of the luma signal may have to be reduced to match with that of the chroma components.
For example, for the 4:2:0 sampling format, the U and V components only have half of the number of samples in vertical and horizontal directions as the luma component. Therefore, 2:1 resolution reduction in vertical and horizontal directions has to be applied to the reconstructed luma samples. The resolution reduction can be achieved by down-sampling process or sub-sampling process.
The down-sampling process in this disclosure refers to signal decimation by applying proper low-pass filtering to reduce possible signal aliasing before reducing resolution (also called decimation in the art). On the other hand, sub-sampling process performs direct decimation without prior filtering. Both the down-sampling process and the sub-sampling process can reduce sampling resolution. While the sub-sampling processing may cause signal aliasing, the sub-sampling processing is used in some signal processing systems due to its simplicity. For chroma intra prediction according to the LM mode, the sub-sampling process is used in horizontal direction and the down-sampling process is used in vertical direction.
In the current HEVC system, the parameter derivation has always assumed the underlying chroma component uses the 4:2:0 sampling format. It is desirable to develop flexible LM-based chroma intra prediction that can adaptively accommodate other chroma sampling formats.